Abstract

The modulating effects of Ca$^2+$ on single K$^+$ channel
currents in canine heart sarcoplasmic reticulum were studied using
a planar lipid bilayer technique. The open-state probability and
the unitary open-state current both decreased gradually as the Ca$^2+$
concentration was reduced from pCa 3 to pCa 7.5. Each single-channel
I-V curve was ohmic at any pCa: the modulating effect of Ca$^2+$
within this range was voltage independent. The Ca$^2+$ dose-response
curves for the conductances and open probabilities were all biphasic
in shape for both sides of the channel at the voltages used. However,
Ca$^2+$ within the pCa ranges used caused significantly more
prominent activation of conductance and gating properties on the
cytoplasmic side than it did on the SR luminal side. Furthermore,
conductance decreased when cytoplasmic Ca$^2+$ concentrations
were greater than pCa 3. The I-V relation in this instance exhibited
inward rectification caused by a voltage-dependent fast block. This
suggests that cardiac SR K$^+$ channel currents may be activated
or inhibited through various types of Ca$^2+$ binding sites on
and within the channels.